Method for conditioning thin sheets of a thermoplastic material to improve windability



Oct. 30, 1962 H. o. CORBEIT 3,060,515

METHOD FOR CONDITIONING THIN OF A THERMOPLASTIC MATERIAL TO IMPROVE WINDABILITY Filed June 10, 1960 FROM PROCESS FIG.2

HERBERT O. CORBETT INVENTOR.

3,059,515 Patented Oct. 30, 1962 ice 3,060,515 METHOD FOR CONDITIONING THIN SHEETS OF A THERMOPLASTIC MATERIAL TO IMPROVE WINDABILITY Herbert 0. Corbett, Canandaigua, N.Y., assignor to National Distillers and Chemical Corporation, New York, N.Y., a corporation of Virginia Filed June 10, 1960, Ser. No. 35,343 Claims. (CI. 18-48) The present invention relates to a method for producing smooth finished rools of thin plastic materials. More particularly this invention relates to a method of overcoming the effects of improper tension control in the winding of thin film webs, and the effects produced by shrinkage and density changes in the film as wound.

In the winding of thin plastic films into rolls, as usually required for use in automatic dispensing and packaging machinery, it is extremely difficult to adjust the winding tension. Furthermore, even though this initial winding tension is originally and continuously established, to produce a roll of initially good appearance, other factors produce abnormal conditions in the wound film. These conditions result, primarily, from the effects of shrinkage and density change, normal to films of any gauge, but particularly critical in films having a gauge less than .002 inch, and particularly less than .001 inch.

When films of these densities are wound and stored for a period of more than one-half hour, the shrinkage, and density increase, characteristically inherent in the material, produce forces which cause the wound film roll to develop a series of small depressed areas and/ or rings or ridges circumferentially of the roll. Such irregularities in the roll of a thin film detracts from the appearance of the roll and, where the roll is stored for some extended period of time, may result in permanent deformation of the film web. This is especially true if the deformation exceeds the elastic limit of the material.

It is an object of the present invention to compensate for characteristic shrinkage and density change in the film material, whereby to avoid the development of depressions or ridges in the wound film. It is also an object of this invention to produce a film roll wherein the film material will substantially maintain a uniformly smooth and finished surface appearance, even after storage for extended periods. It is a further object of the invention to avoid permanent deformation of a thin thermoplastic film material as a result of excessive tension exerted thereon by shrinkage and density changes occurring subsequent to being wound under the initial tension required for micetive roll winding procedures.

The invention and its objects may be more fully understood from the following description, when it is read with reference to the accompanying drawings, wherein:

FIG. 1 is a diagrammatic view in side elevation of a film winding system as contemplated by the present invention; and

FIG. 2 is an enlarged showing of a pair of nip rolls as employed in FIG. 1.

In the drawings, like parts are designated by the same numerals.

Referring now to FIG. 1, in this figure is shown an end view of a roll winding shaft 1, adapted to receive a roll center tube 2, on which is wound a web of a thermoplastic material 3. The web 3 is derived from a conventional source (not shown), as by extrusion in either a sheeted, or lay-flat tubular form. On occasion, the film indicated in the drawing may be a wide sheet previously folded, one or more times, in a longitudinal direction.

The film 3 is led under, around, and over a first nip roll 4. A second nip roll 5 is disposed for pressure contact with the roll 4, and the film 3 is passed therebetween,

so as to move under, around and over the roll 5. In the path followed by the film, it is in contact with each of the nip rolls 4 and 5 for at least of their respective circumferential surfaces.

Leaving the roll 5, the film passes under an idler, guide roll 6. Preferably, the roll 6 is a dancer roll. This is one which is resiliently mounted so as to apply a substan tially uniform tension to the web as it is wound upon the center tube 2, by the shaft 1.

The shaft 1 and the nip rolls 4 and 5 are driven, by means not shown. Preferably the winding shaft 1, and the ni rolls are driven at a uniform or synchronous rate of speed, and by the same drive means. In any event, the dancer roll 6 is adjusted so as to compensate for any slight speed variation, while maintaining a substantially uniform tension in the web as wound.

In FIG. 2, the nature of the nip rolls 4 and 5 are more clearly shown. The roll 4 is formed by a relatively thick cylinder of a resilient material, such as 70 durometer rubber or equal, mounted or coated on a steel center shaft 4a. The material preferably contemplated is that defined per ASTM standard D 676-55 T-Shore type A. The roll 5 is of steel, having a shaft 5a and is embossed, as by conventional engraving methods, and then chrome plated and polished. In FIG. 2, a typical embossing pattern is illustrated, wherein by engraving the roll surfac to form helical grooves 7 and 8 of opposite rotation, and to form a series of circumferential grooves 9 spaced longitudinally of the roll at points of intersection of the helical grooves, a pattern of equilateral triangular bosses is provided, wherein these bosses extend in alternate base to base, and apex to apex relationship in alternate, and oppositely arranged rows, longitudinally of the roll. A similar result may be obtained by embossing the roll to provide a series of ridges corresponding to the grooves 7, 8 and 9 shown in FIG. 2.

Also shown by FIG. 2, is one means for uniform, driven rotation of the rollers 4 and 5. As shown, the shafts 4a and 5a are equipped with meshing spur gears 10 and 11 respectively, and the shaft 4a is additionally provided with a spur gear 12. The spur gear 12 may be connected in a driving gear train to activate the rolls 4 and 5 through the gears 10 and 11. If desired, of course, the gear 12 may be replaced by a sprocket and sprocket chain drive, a pulley and belt drive, or the shaft 4a may be directly driven as from the shaft of an electric motor. In FIG. 2, shaft support means, as well as means for adjusting and maintaining an interacting pressure between the rolls 4 and 5 is not shown. Such means of support, and means for holding the rolls under pressure are well known in the art, and do not require any distinctive modification to permit their employment in the manner contemplated according to the present invention.

In the method and operation contemplated according to the present invention, a thin film Web being Wound, as on the roll center tube 2, is embossed immedaitely prior to being laid up on the roll. Embossing of the film is accomplished by pressing the film between a smooth resilient surface, and a non-resilient embossed surface. As shown in the drawings, this may be acomplished immediately prior to rolling of the film, by passing the film over and between two nip rolls such as the rolls 4 and 5, wherein the one roll (4) presents a smooth, resilient surface, and the other roll (5) presents a non-resilient, embossed surface.

Pressure on and between the rolls 4 and 5 is adjusted and maintained at a level sufficient to press the embossed areas of the non-resilient roll into the surface of the resilient roll to a degree sufiicient to impress the embossed roll pattern into the material of the film web, without exceeding the elastic limit of the web material. By this procedure, the web as wound on the roll is distorted in a regular pattern of raised and depressed areas which tend to reduce the length and width of the web, while retaining a uniformly distributed excess of material in each layer as wound upon the roll.

If the elastic limit of the web material is not exceeded, the elastic memory of the material will permit the material to return to its original smooth sheet condition after a certain period of rest, or by the applied force of internal tension. Such tension force will result from the characteristic shrinkage and material density changes which occur in the material as it ages and sets. The additional material made available in each Web layer on the roll by the embossing step, compensates for the effects of characteristic shrinkage and density change therein, preventing the formation of wrinkle ridges, and depressions in the finished, rolled web. As employed in this description, the terms elastic limit and elastic memory may be defined in the manner according to A.S.T.M. D638-52T, page 32.

In the rolling of thin web materials, having a gauge less than .002 of an inch, the grooves, ridges, and bosses provided on the surface of the embossing roller preferably are limited to a vertical or radial dimension in the range of from about 0.002 to about 0.0005 of an inch, with the ridges or grooves having a lateral dimension of from about & to about Ms of an inch. In addition, the spacing of the grooves or ridges on the embossed roll surface is arranged to form not substantially less than about 64 raised or depressed bosses per square inch in the treated web material. A preferred technique is to form between about 1024 and about 64 bosses per square foot. Under such conditions the web material may be embossed at pressures between the rollers in the range of from about 25 to about 150 pounds per square inch. 1

What is claimed is:

1. In the forming of rolls from sheets of thin thermoplastic materials, a method of preconditioning said materials whereby to improve the finished apearance of said rolls after storage which comprises temporarily embossing said sheets to produce a regular embossed pattern in said sheet immediately prior to laying said sheet upon a roll thereof, and under conditions such as to avoid distortion of the film material in excess of its elastic limit, and to produce a uniformly distributed excess of said thermoplastic material in each layer on said roll, which excess is substantially compensatory for overall longitudinal and lateral dimensional reduction of the material in each layer produced by shrinkage and density change therein.

2. A method according to claim 1, wherein said regular embossed pattern comprises a series of raised bosses separated by intersecting channel-like areas on one side of said sheet, and on the opposite side thereof, a series of intersecting ridges separating depressed areas enclosed thereby.

3. A method according to claim 2, wherein said channel-like areas and intersecting ridges are of a vertical dimension substantially in the range of from about .0002 to about .0005 of an inch, and a lateral dimension of from about 6 to about A3 of an inch.

4. A method according to claim 2, wherein said raised bosses and depressed areas form a pattern which includes from about 1024 to about 64 such bosses and areas per square foot.

5. A method according to claim 1, wherein said embossed pattern is impressed on said sheet under a pressure in the range of from about 25 to about 150 pounds per square inch.

References Cited in the file of this patent UNITED STATES PATENTS 2,609,568 Getchell Sept. 9, 1952 2,976,567 Jones et al Mar. 28, 1961 FOREIGN PATENTS 550,534 Great Britain Ian. 13, 1943 

